A waveform equalizing device for equalizing the waveform of a digital modulated signal generally uses a finite impulse response (FIR) filter and an infinite impulse response (IIR) filter to perform equalization. To improve the equalization performance, a decision feedback equalizer (DFE) is used as the IIR filter. The DFE however generates error propagation and thus may cause degradation in equalization performance.
To address the above problem, Patent Document 1 discloses an equalizer in which a trellis decoder permitting use of a traceback path is employed to make value decision to thereby reduce error propagation and input data is delayed by a delay having arisen with the traceback path operation at the time of the value decision. Patent Document 2 discloses a device in which a DFE is coupled to a plurality of path memory outputs of a trellis decoder to allow the DFE to receive firm decision results and thus reduce error propagation. In this way, by use of a trellis decoder, high-precision waveform equalization has been attained for a signal containing Gaussian noise.    Patent Document 1: International Publication No. WO2002/084965    Patent Document 2: International Publication No. WO2002/087180